Multi-layer diffraction type polarizer and liquid crystal element

ABSTRACT

A multi-layer diffraction type polarizer and a liquid crystal element capable of realizing a stable high extinction ratio is provided. Further, a liquid crystal element is obtained, which can rotate the polarization direction maintaining high linearity of output light when linearly polarized light is incident.  
     A multi-layer diffraction type polarizer is formed by laminating at least two polarizing diffraction gratings each having a birefringent material which straightly transmits incident light having a first polarization direction without functioning as a diffraction grating, and diffracts incident light having a second polarization direction perpendicular to the first polarization direction by functioning as a diffraction grating.  
     Further, in order to realize an optical attenuator having a high extinction ratio even at low voltage, a phase plate made of an organic thin film is provided to cancel the retardation of the liquid crystal cell remaining when the voltage is applied. Further, in order to rotate the polarization direction of a linearly polarized incident light, the liquid crystal cell is provided with a λ/4 phase plate comprising an organic thin film.

TECHNICAL FIELD

[0001] The present invention relates to a multi-layer diffraction typepolarizer and a liquid crystal element, in particular, a multi-layerdiffraction type polarizer used as an isolator for opticalcommunications and a liquid crystal element used for an opticalattenuator or for a polarization rotator.

BACKGROUND ART

[0002] In an information reading optical head device for an optical disksuch as a CD or a DVD, for example, a polarizing diffraction grating 500shown in FIG. 15 is used as a polarizing beam splitter. The polarizingdiffraction grating comprises a diffraction grating 1 made of abirefringent material layer having an ordinary refractive index n_(o)and an extraordinary refractive index n_(e) (n_(o)≠n_(e)) formed on oneside of a glass substrate which is a transparent substrate 4, and thediffraction grating 1 has a periodical structure of concavo-convex shapewith a step height d in cross section.

[0003] The concavo-convex portion of the periodical structure is filledwith a homogeneous refractive index transparent material 3 having arefractive index n_(s) substantially equal to the ordinary refractiveindex n_(o) so that the concavo-convex portion is leveled, and a glasssubstrate as a transparent substrate 5 is overlaid on the homogeneousrefractive index transparent material 3 to form the polarizingdiffraction grating 500. Here, |n_(e)−n_(s)|×d is made to be a half ofthe wavelength λ of incident light, whereby a polarizing diffractiongrating is obtained, in which an ordinary polarized incident light(polarized in the direction providing ordinary refractive index) isstraightly transmitted without being diffracted, and an extraordinarypolarized incident light (polarized in the direction providingextraordinary refractive index) is diffracted and is not straightlytransmitted.

[0004] There has been a problem that a sufficient extinction ratio cannot be obtained when such a polarizing diffraction grating is used as anisolator for optical communication using a wavelength band of 1400 to1700 nm. Namely, provided that the intensity of a first linearlypolarized light (for example, ordinary polarized light) straightlytransmitted is I₁ and the intensity of a second linearly polarized light(extraordinary polarized light) straightly transmitted and polarized ina direction perpendicular to the polarization direction of the firstlinearly polarized light is I₂, a ratio I₂/I₁ (hereinafter referred toas extinction ratio) of light of given single wavelength λ₀ becomes atmost −20 dB. However, since the transmittance of the straightlytransmitted light of extraordinary polarized light is expressed bycos²(0.5×π×λ₀/λ), component of the incident light straightly transmittedwithout being diffracted is increased and the extinction ratio isdeteriorated as the wavelength λ is away from λ₀.

[0005] Further, in order to achieve a higher extinction ratio for agiven single wavelength, it is necessary to accurately form the stepheight d of the periodical structure having a concavo-convex shape, andit has been difficult to obtain a polarizing diffraction grating havinga high extinction ratio with good reproducibility.

[0006] Further, an example of a conventional optical attenuatoremploying liquid crystal is shown in FIG. 16. The optical attenuator isconstituted by a liquid crystal cell 210 comprising transparentsubstrates 15 and 16 on which transparent electrodes 13 and 14 areformed, and a liquid crystal layer 11 of nematic liquid crystal in whichthe alignment direction of liquid crystal molecules is in parallel withthe substrate surfaces and at an angle of 45° to the X-axis, sandwichedbetween the transparent substrates 15 and 16 and sealed inside a sealingmember 18 provided at the peripheries of the substrates; and a polarizer9 disposed at the light output side of the liquid crystal cell, whichtransmits only linearly polarized light polarized in X-axis direction.

[0007] Here, an AC power source 19 is connected to the transparentelectrodes 13 and 14 to supply rectangular waves, and the thickness ofthe liquid crystal layer 11 is determined so that the retardation valueof the liquid crystal cell 210 becomes about λ/2 for a linearlypolarized light having a wavelength λ and polarized in the direction ofY-axis, when the voltage is not applied by the power source. Here, thepurpose of setting the retardation value of the liquid crystal layer 11to be about λ/2, is to minimize the insertion loss of the opticalattenuator when the voltage is not applied, and to make the opticalattenuator function as a λ/2 plate.

[0008] In this optical attenuator, the linearly polarized lightpolarized in the direction of Y-axis transmitted through the liquidcrystal layer when the voltage is not applied between the transparentelectrodes, becomes a linearly polarized light polarized in thedirection of X-axis and is transmitted through the polarizer. When thevoltage is applied, the alignment direction of liquid crystal moleculesare tilted in the direction of the thickness of the liquid crystallayer, namely tilted perpendicularly to the substrates, as the appliedvoltage is increased. Accordingly, the retardation value of the liquidcrystal cell is decreased and the light transmitted through the liquidcrystal cell 210 becomes an elliptically polarized light. As a result,since the intensity of the transmitted light through the polarizer issimply decreased by the increase of the applied voltage, the opticalattenuator is of a voltage variable type.

[0009] In a case of an optical attenuator employing a liquid crystalelement, for optical communications using incident light having awavelength of, for example, 1300 to 1600 nm, it is necessary to make theliquid crystal layer thicker than that of an optical attenuator for avisible wavelength region in order to make the retardation value of theliquid crystal cell to be λ/2. As a result, there has been a problemthat a polarized light component transmitted through the polarizerremains, and therefore, an optical attenuator having a high extinctionratio can not be obtained, since even if an AC voltage having a voltageamplitude of at least 10 V is applied, the alignment direction of theliquid crystal molecules is not sufficiently oriented in the directionof the thickness of the liquid crystal layer and the retardation valueof the liquid crystal cell does not become zero.

[0010] Further, FIG. 17 shows an example of conventional liquid crystalelement for rotating the polarization direction of incident light as alinearly polarized light in accordance with the magnitude of an appliedvoltage.

[0011] The liquid crystal element is constituted by a liquid crystalcell 210 comprising transparent substrates 15 and 16 on whichtransparent electrodes 13 and 14 are formed, a liquid crystal layer 11of nematic liquid crystal in which the alignment direction of liquidcrystal molecules is in parallel with the substrate surfaces and in thedirection at 45° to X-axis, the liquid crystal layer being sandwichedbetween the substrates and sealed by a sealing member 18; and a phaseplate 10 made of a birefringent crystal having a fast axis or a slowaxis in the direction of X-axis disposed at the light output side of theliquid crystal cell 210. Here, an AC power source 19 for generatingrectangular waves is connected to the transparent electrodes 13 and 14,the thickness of the liquid crystal layer 11 is determined so that theretardation value R of the liquid crystal cell 210 for the linearlypolarized incident light having a wavelength and polarized in thedirection of X-axis when the voltage is not applied, is substantiallyλ/2, and the retardation value of the phase plate 10 is λ/4.

[0012] In this liquid crystal element, when the voltage is not appliedbetween the transparent electrodes 13 and 14, the light transmittedthrough the liquid crystal layer becomes a linearly polarized lightpolarized in the direction of Y-axis, and is transmitted through thephase plate maintaining the state of linear polarization in thedirection of Y-axis since the polarization direction coincides witheither the slow axis or the fast axis of the phase plate 10. As theapplied voltage is increased, the alignment direction of the liquidcrystal molecules is tilted in the direction of the thickness of theliquid crystal layer. Accordingly, the retardation value R of the liquidcrystal layer is decreased and the light transmitted through the liquidcrystal cell 210 becomes an elliptically polarized light. Here, thepolarization direction is rotated in accordance with the retardationvalue R of the liquid crystal layer maintaining the state of linearpolarization when the light is transmitted through the phase plate 10.

[0013] The phase plate 10 to be employed for such a liquid crystalelement is generally a birefringent crystal such as a quartz processedto have a thickness of at least 0.3 mm. However, in the case of abirefringent crystal, there has been a problem that the retardationvalue depends strongly on the incident angle as an angle between thepropagation direction of the incident light and the normal line of thephase plate, which causes variation of the retardation value in thedevice plane for converging rays or diverging rays, and accordingly,polarization of the output light is not consistent. Further, since theretardation value has a dependency on wavelength, there has been aproblem that when the incident light has a bandwidth in the wavelength,the linearity of the linearly polarized incident light is deterioratedwhen it is output from the element.

[0014] Considering the above-mentioned circumstances, it is an object ofthe present invention to provide a multi-layer diffraction typepolarizer and a liquid crystal element capable of realizing a stable andhigh extinction ratio.

[0015] Further, considering the above-mentioned circumstances, it isanother object of the present invention to provide a liquid crystalelement for rotating the polarization direction of a linearly polarizedlight incident on the device and outputting the light maintaining thehigh linearity.

DISCLOSURE OF THE INVENTION

[0016] The present invention provides a multi-layer diffraction typepolarizer comprising a lamination of at least two polarizing diffractiongratings each comprising a birefringent material, wherein thediffraction gratings each straightly transmits incident light having afirst polarization direction without functioning as a polarizer, anddiffracts incident light having a second polarization directionperpendicular to the first polarization direction by functioning as apolarizer.

[0017] Further, the present invention provides the multi-layerdiffraction type polarizer, wherein each of the polarizing diffractiongratings comprises a birefringent material layer formed on a transparentsubstrate and having an ordinary refractive index of n_(o) and anextraordinary refractive index of n_(e) (n_(o)≠n_(e)), the birefringentmaterial layer being processed to have a periodical concavo-convex shapehaving a step height of d in cross section, a homogeneous refractiveindex transparent material having a refractive index equal to n_(o) orn_(e) is filled in at least the concave portions, and the retardationvalue |n_(e)−n_(o)|×d is (m+½) times (m is zero or a positive integer)the wavelength λ of the incident light.

[0018] Further, the present invention provides the above multi-layerdiffraction type polarizer, wherein the step heights d of the polarizingdiffraction gratings are different from each other.

[0019] Further, the present invention provides a liquid crystal elementcomprising a liquid crystal cell comprising transparent substrateshaving electrodes and a liquid crystal layer sandwiched between them,the liquid crystal cell having a retardation value for a linearlypolarized light having a wavelength of λ incident and transmittedthrough the liquid crystal cell, the retardation value changing from R₁to R₂ (R₁>R₂>0) when the voltage applied between the electrodes ischanged from V₁ to V₂ (V₁≠V₂) ; and a phase plate having a retardationvalue R for a linearly polarized light having a wavelength of λ, theretardation value R satisfying a relation R+R_(v)=m×λ (m: integer) witha retardation value R, generated by the voltage satisfying R₁≧R_(v)≧R₂.

[0020] Further, the present invention provides a liquid crystal element,wherein the liquid crystal in the liquid crystal element is a nematicliquid crystal, and the alignment of the liquid crystal molecules is aparallel alignment in which the liquid crystal molecules are aligned inparallel in one direction between the transparent substrates when thevoltage is not applied, the first phase plate satisfies a relationR+R_(v)=0, and the fast axis direction of the first phase plate iswithin an angle of 45° to the slow axis direction of the liquid crystallayer.

[0021] Further, the present invention provides the liquid crystalelement, which further comprises a polarizing diffraction gratingcomprising a birefringent material at at least one of the light inputside or the light output side of the liquid crystal element, wherein thediffraction grating straightly transmits incident light having a firstpolarization direction without functioning as a polarizer, and diffractsincident light having a second polarization direction perpendicular tothe first polarization direction by functioning as a polarizer.

[0022] Further, the present invention provides a liquid crystal elementhaving a λ/4 phase plate, comprising a liquid crystal cell comprisingsubstrates having electrodes and a liquid crystal layer sandwichedbetween them, the liquid crystal cell having a retardation valuechangeable for incident linearly polarized light having a wavelength ofλ, depending on the magnitude of a voltage applied between theelectrodes; and a λ/4 phase plate producing a phase-shift correspondingto a retardation value of substantially λ/4 for the linearly polarizedincident light, the λ/4 phase plate having an organic thin film and thealignment direction of molecules constituting the organic thin film isin parallel with the plane of the phase plate;

[0023] wherein the fast axis direction of the λ/4 phase plate is at anangle of about 45° to the fast axis direction of the liquid crystalcell.

[0024] Further, the present invention provides the above liquid crystalelement having a λ/4 phase plate, wherein the phase plate comprises aliquid crystal polymer as the organic thin film, and the phase plate andthe liquid crystal cell are integrally formed.

[0025] Further, the present invention provides the above liquid crystalelement having a λ/4 phase plate, wherein the phase plate comprises atleast two liquid crystal polymer layers, the retardation values of thetwo liquid crystal polymer layers are different from each other, and thefast axis directions or the slow axis directions of the two liquidcrystal polymer layers are different from each other.

[0026] Further, the present invention provides the above liquid crystalelement having a λ/4 phase plate, which comprises the liquid crystalcell, a first liquid crystal polymer layer and a second liquid crystalpolymer layer arranged in this order from light-input side, wherein withrespect to the center wavelength λ of incident light, the retardationvalue of the first liquid crystal polymer layer is substantially λ/2,and the retardation value of the second liquid crystal polymer layer issubstantially λ/4; and with respect to the polarization direction of theincident light, the fast axis direction of the first liquid crystalpolymer layer and the fast axis direction of the second liquid crystalpolymer layer are about 30 degrees and about −30 degrees respectively,or otherwise, the slow axis direction of the first liquid crystalpolymer layer and the slow axis direction of the second liquid crystalpolymer layer are about 30 degrees and about −30 degrees respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a side view showing an example of the construction of amulti-layer diffraction type polarizer of the present invention.

[0028]FIG. 2 is a side view showing the function of the multi-layerdiffraction type polarizer shown in FIG. 1 when an extraordinarypolarized light is incident.

[0029]FIG. 3 is a side view showing the function of the multi-layerdiffraction type polarizer shown in FIG. 1 when an ordinary polarizedlight is incident.

[0030]FIG. 4 is a plan view showing an example of two types ofdiffraction grating patterns constituting the multi-layer diffractiontype polarizer shown in FIG. 1.

[0031]FIG. 5 is a side view showing an example of an optical system forseparating straightly transmitted light and diffraction light,comprising the multi-layer diffraction type polarizer shown in FIG. 1.

[0032]FIG. 6 is a plan view showing an example of focusing positions ofstraightly transmitted light and diffracted light of the lighttransmitted through the multi-layer diffraction type polarizer shown inFIG. 1, on the focal plane of a condenser lens.

[0033]FIG. 7 is a graph showing an example of the wavelength-dependency(calculated) of the transmittance for the extraordinary polarized lightof the multi-layer diffraction type polarizer of the present invention.

[0034]FIG. 8 is a side view showing an example of the construction ofthe liquid crystal element of the present invention.

[0035]FIG. 9 is a plan view showing the relation between the slow axisdirection of the liquid crystal element and the fast axis direction ofthe phase plate of the present invention.

[0036]FIG. 10 is a side view showing an example of the construction of acomplex type liquid crystal element of the present invention wherein themulti-layer diffraction type polarizer and the liquid crystal device areintegrally formed.

[0037]FIG. 11 is a graph showing the relations between the extinctionratio and the voltage applied to the liquid crystal cell in the complextype liquid crystal element of the present invention and a conventionalliquid crystal element.

[0038]FIG. 12 is a side view showing an example of another constructionof the complex type liquid crystal element of the present invention.

[0039]FIG. 13 is a view showing the relations among the fast and theslow axes of the liquid crystal, those of the phase plate constitutingthe liquid crystal element of the present invention and polarizationdirections.

[0040]FIG. 14 is a side view showing another example of the constructionof the liquid crystal element of the present invention.

[0041]FIG. 15 is a side view showing an example of the construction of aconventional diffraction type polarizer.

[0042]FIG. 16 is a side view showing an example of the construction of aconventional optical attenuator.

[0043]FIG. 17 is a side view showing an example of the construction of aconventional polarization rotator.

BEST MODE FOR CARRYING OUT THE INVENTION

[0044] The present invention provides a multi-layer diffraction typepolarizer comprising a lamination of at least two polarizing diffractiongratings, each having a birefringent material which straightly transmitsincident light having a first polarization direction without functioningas a diffraction grating, and diffracts incident light having a secondpolarization direction by functioning as a diffraction grating. Such aconstruction produces an effect of increasing the extinction ratio.

First Embodiment of the Multi-Layer Diffraction Type Polarizer

[0045]FIG. 1 is a side view showing a first embodiment of theconstruction of the multi-layer diffraction type polarizer of thepresent invention. On one side of each of a transparent substrate 4 anda transparent substrate 5, a birefringent material layer having anordinary refractive index n_(o) and an extraordinary refractive indexn_(e) (n_(o)≠n_(e)), is formed so that its fast axis (the direction inwhich the refractive index becomes the ordinary refractive index) is inthe direction of X-axis in FIG. 1. Then, the birefringent materiallayers are processed to be a diffraction grating 1 having a periodicalstructure whose cross-sectional shape is a concavo-convex shape with astep height of d₁ and a grating pitch of p₁, and a diffraction grating 2having a periodical structure whose cross-sectional shape is aconcavo-convex shape with a step height of d₂ and a grating pitch of p₂.

[0046] Then, at least convex portions of them are filled with ahomogeneous refractive index transparent material 3 having a refractiveindex n_(s) (which equals to the ordinary refractive index n_(o) or theextraordinary refractive index n_(e)) to form polarizing diffractiongratings on the transparent substrate 4 and the transparent substrate 5,and thereafter, the transparent substrate 4, the transparent substrate 5and the transparent substrate 6 are laminated to form a multi-layerdiffraction type polarizer 100. Here, “at least convex portions” meansthat either only the convex portions are filled or both the concavo andconvex portions are filled to be covered. Here, the homogeneousrefractive index transparent material means a transparent material whoserefractive index is isotropic. The longitudinal directions as thedirection of grooves of the convex portions of the grating on thetransparent substrate 4 and that of the transparent substrate 5 may bein parallel or perpendicular or at a predetermined angle to each other.Since diffracted light produced by the diffraction grating is in adirection perpendicular to the longitudinal direction of the grating, itis possible to produce the diffracted light in a desired direction bymaking the longitudinal directions of the diffraction grating 1 and thediffraction grating 2 to have a predetermined angle.

[0047] Here, it is preferred, for example, to employ a homogeneousrefractive index transparent material 3 having a refractive index n_(s)substantially equal to the ordinary refractive index n_(o), and to makethe step heights d₁ and d₂ so that each of the retardation values|n_(e)−n_(s)|×d₁ and |n_(e)−n_(s)|×d₂ becomes (m+½) times (m is 0 or apositive integer) the wavelength of incident light from the followingreasons. The reason is that the intensity of the straightly transmittedlight of the incident light having the second polarization directionthereby becomes minimum, and a high extinction ratio can be obtained.Here, the (m+½) includes a magnification range of within ±10% from(m+½), since the effect of the present invention does not change in thisrange.

[0048] When an extraordinary polarized light (S polarized light:polarized light vibrating in a direction perpendicular to the surface ofthe drawing) is incident on such a multi-layer diffraction typepolarizer 100, each of the polarizing diffraction gratings, i.e. adiffraction grating 1 and a diffraction grating 2 of the presentinvention, functions as a diffraction grating having a periodicaldistribution of refractive index n_(e) and refractive index n_(s)derived from the periodical structure of concavo-convex shape to producediffracted light. Hereinafter, the diffraction grating 1 means apolarizing diffraction grating 1 comprising the diffraction grating 1,and the diffraction grating 2 means in the same manner.

[0049] In order to reduce the dependency of the diffraction efficiencyon wavelength and to reduce the step height to be formed in themulti-layer diffraction type polarizer, each of the retardation values|n_(e)−n_(s)|×d₁ and |n_(e)−n_(s)|×d₂ is preferably made to be ½ timesthe wavelength of the output light (which corresponds to m=0). Here, the½ times includes a magnification variation of within ±10% in the samemanner as described above. Namely, the magnification may be within arange of from 0.55 to 0.45.

[0050] Here, since a part of an extraordinary polarized light (Spolarized light) straightly transmitted through the diffraction grating2 without being diffracted is diffracted by the diffraction grating 1,the extraordinary polarized light straightly transmitted through themulti-layer diffraction type polarizer 100 is minimized.

[0051] On the other hand, when an ordinary polarized light (P polarizedlight: polarized light vibrating in the direction parallel to thedrawing surface) is incident into the multi-layer diffraction typepolarizer 100, the diffraction grating 1 and the diffraction grating 2of the present invention become equivalent to media having therefractive index n_(s) even if they have concavo-convex periodicalstructures, and therefore, the incident light is straightly transmittedwithout being diffracted.

[0052] Therefore, by laminating the diffraction grating 1 and thediffraction grating 2 each having a transmittance of at least 90% forordinary polarized light and a transmittance of at most 5% forextraordinary polarized light having a polarization directionperpendicular to the polarization direction of the ordinary polarizedlight, a multi-layer diffraction type polarizer can be obtained whichmakes straightly transmitted light at most 0.5% of the incident light ifthe extraordinary polarized light is incident.

[0053] Here, as shown in two types of diffraction grating patterns inFIG. 2 and in FIG. 1, the diffraction grating 1 is formed to be a lineargrating having a grating pitch of p₁ and to have an angle of θ₁ betweenlongitudinal direction of the grating and the X-axis, and thediffraction grating 2 is formed to be a linear grating having a gratingpitch of p₂ and to have an angle of θ₂ between the longitudinaldirection of the grating and the X-axis.

[0054] In general, when light diffracted by the diffraction grating 2 isdiffracted again by the diffraction grating 1 and superposed on thestraightly transmitted light, a straightly transmitted component of thelight is increased and the extinction ratio is deteriorated as a result.However, by making the grating pitch p₁ and the grating pitch p₂different from each other or by making the angle θ₁ and the angle θ₂ ofthe grating longitudinal directions different from each other, such adeterioration of the extinction ratio can be prevented. Namely, thediffraction grating 1 and the diffraction grating 2 as the constituents,are preferably formed so that their grating pitches or theirlongitudinal directions do not coincide with each other, wherebymulti-diffraction light by the diffraction grating 1 and the diffractiongrating 2 do not superpose on the straightly transmitted light, and theextinction ratio is not deteriorated. Even if the step heights d₁ and d₂of the diffraction gratins are equal, the extinction ratio is notdeteriorated as long as p₁ and p₂ or θ₁ and θ₂ are different from eachother.

[0055]FIG. 5 is a side view showing an example of the construction ofthe optical system of an isolator having a high extinction ratioemploying the multi-layer diffraction type polarizer 100 of the presentinvention. A parallel light as a mixture of the ordinary polarized lightand the extraordinary polarized light is incident on the multi-layerdiffraction type grating 100, and a condenser lens 7 is provided at theoutput side, whereby the ordinary polarized light straightly transmittedthrough the multi-layer diffraction type polarizer 100 is focused on thefocal plane on the optical axis of the condenser lens 7. On the otherhand, the extraordinary polarized light diffracted by the multi-layerdiffraction type polarizer 100 is focused on the focal plane off theoptical axis of the condenser lens 7.

[0056] Accordingly, by providing an aperture 8 having an opening on thefocal plane on the optical axis of the condenser lens 7, an isolatorwhich transmits only ordinary polarized light and shutters extraordinarypolarized light, is formed. Here, by providing a photodetector having aphoto acceptance area corresponding to the opening instead of theaperture 8, only the ordinary polarized light component can be detected.Further, by providing the core of an optical fiber for transmittinglight instead of the opening of the aperture, only the ordinarypolarized light can be transmitted.

[0057]FIG. 6 shows an example of focusing positions of straightlytransmitted light and diffracted light formed on the focal plane of thecondenser lens 7 in FIG. 5 in a case that θ₁=θ₂=0° and p₂ is twice aslarge as p₁ when a diffraction grating 1 having an angle θ₁ in thelongitudinal direction of the grating and a grating pitch of p₁ and adiffraction grating 2 having an angle θ₂ in the longitudinal directionof the grating and a grating pitch of p₂, are employed.

[0058] The ordinary polarized light (P polarized light) is notdiffracted by the diffraction grating 1 and the diffraction grating 2(the 0th order diffraction light becomes again the 0th order diffractionlight) and focused at the position indicated by ⊚ on the optical axis.

[0059] This is referred to as 0th×0th.

[0060] Further, extraordinary polarized light beams (S polarized light)diffracted by the diffraction grating 1 and the diffraction grating 2 asdiffracted light beams of the same sign and the same order number (±1storder diffraction light beams are further diffracted and all of thembecome ±1st order diffraction light beams) are focused at positionsdesignated by Δ or ∇. They are designated as 1st×1st and −1st×−1st, andvice versa.

[0061] Further, extraordinary polarized light beams diffracted by thediffraction grating 1 as ±1st order light beams but not diffracted bythe diffraction grating 2 and transmitted as 0th order diffracted lightbeams, are focused at positions designated by ◯.

[0062] Further, extraordinary polarized light beams diffracted by thediffraction grating 2 as ±1st order light beams but not diffracted bythe diffraction grating 1 (0th order diffracted light beams), andextraordinary polarized light beams diffracted by the diffractiongrating 1 and the diffraction grating 2 as different signs and differentorder numbers (a +1st order diffracted light beam is diffracted as a−1st order diffracted light beam, or a −1st order diffracted light beamis diffracted as a +1st order diffracted light beam), are focused atpositions designated as □.

[0063] The diffraction direction of an extraordinary polarized lightbeam is determined by the grating longitudinal direction angles θ₁ andθ₂ of the diffraction grating 1 and the diffraction grating 2, and thedistance of the focusing positions of the diffracted light beams fromthe optical axis is determined by the wavelength of incident light,grating pitches p₁ and p₂, and the focal length of the condenser lens 7.

Second Embodiment of Multi-Layer Diffraction Type Polarizer

[0064] Birefringent material layers constituting the diffraction grating1 and the diffraction grating 2 are preferably formed so that their stepheights d₁ and d₂ are different from each other. Further, provided thatthe wavelength of the incident light is within a range of from λ₁ to λ₂,it is preferred that d₁ and d₂ are present between λ₁/(2×Δn) andλ₂/(2×Δn) which are ratios of the wavelengths λ₁ and λ₂ to thedifference Δn between the ordinary refractive index and theextraordinary refractive index of the birefringent material layer. Bysuch a construction, a relatively high extinction ratio can be obtainedfor incident light having a wide wavelength band.

[0065] The second embodiment of the multi-layer diffraction typepolarizer of the present invention will be described. When anextraordinary polarized light having a wavelength of is incident on themulti-layer diffraction type polarizer of this embodiment, thetransmittance η₀ of the straightly transmitted light (0th orderdiffracted light) not diffracted by the diffraction grating 1 and thediffraction grating 2, is approximately described as η₀=(cos(Φ/2))².Here, Φ=2×π×Δn×d/λ, Δn=|n_(e)−n_(s)|>0, n_(o) and n_(s) areapproximately equal, and d=d₁ in the diffraction grating 1 and d=d₂ inthe diffraction grating 2.

[0066] When the wavelength of the incident light is within a range offrom λ₁ to λ₂, it is effective to make values of d₁ and d₂ differentfrom each other within a range of from λ₁/(2×Δn) and λ₂/(2×Δn) in orderto achieve high diffraction efficiency in this wavelength band. When thewavelength of the incident light is within a range of from 1400 to 1700nm, and periodic structures of concavo-convex shape can be formed tohave step heights of d₁=4.8 μm and d₂=5.5 μm employing a birefringentmaterial of Δn=0.15, the wavelength-dependency of straight transmittanceη₀ of the extraordinary polarized light is calculated and the result isshown in FIG. 7. Here, λ₁/(2×Δn)=4.67 μm, λ₂/(2×Δn)=5.67 μm, and d₁ andd₂ are values between them.

[0067] In FIG. 7, the straight transmittances η₀ in each of thediffraction grating 1 and the diffraction grating 2 for theextraordinary polarized light are designated as Δ and □ respectively,the straight transmittance η₀ of the multi-layer diffraction typepolarizer 100 as a whole for extraordinary polarized light is designatedas ◯. The ordinary polarized incident light is scarcely diffracted andat least 90% of the incident light is straightly transmitted, andaccordingly, the multi-layer diffraction type polarizer 100 functions asan isolator having an extinction ratio of at most −35 dB in a wavelengthband of from 1400 to 1700 nm.

[0068] Here, by further laminating multi-layer diffraction typepolarizers 100 of the present invention in series, a still higherextinction ratio can be obtained.

First Embodiment of Liquid Crystal Element

[0069] Then, a first embodiment of the liquid crystal element of thepresent invention will be described.

[0070] The liquid crystal element of the present invention is a liquidcrystal element having the following construction. Namely, the liquidcrystal element comprises a liquid crystal cell comprising transparentsubstrates having electrodes and a liquid crystal layer sandwichedbetween them, and when the voltage applied between the electrodes ischanged from V₁ to V₂ (V₁≠V₂), the retardation value of the liquidcrystal cell for a linearly polarized light having a wavelength of λincident and transmitted, changes from R₁ to R₂ (R₁>R₂>0). Further, theliquid crystal element comprises a phase is plate having a retardationvalue R for the linearly polarized light having a wavelength of λ,satisfying a relation R+R_(v)=m×λ (m: integer) with R, whereR₁≧R_(v)≧R₂.

[0071] By thus constituting the liquid crystal element of the presentinvention, the liquid crystal element can produce a high extinctionratio at a low voltage. Now, the liquid crystal element of the presentinvention will be described in detail with reference to the drawings.

[0072]FIG. 8 is a side view showing an example of the construction ofthe liquid crystal element of the present invention. Transparentelectrodes 13 and 14 are formed on one side of transparent substrates 15and 16 respectively, and on the top of the transparent electrodes 13 and14, alignment layers (not shown) processed to have the same alignmentdirection are formed, and a sealing member 18 is employed to form acell. Further, in the cell, a nematic liquid crystal having an ordinaryrefractive index of n_(o) (1c) and an extraordinary refractive index ofn_(e) (lc) (n_(o)(1c)<n_(e)(1c)) is injected to form a liquid crystallayer 11, whereby a liquid crystal cell 210 in which the direction ofliquid crystal molecules are aligned in parallel with the substrates canbe obtained.

[0073] Further, the surfaces of the transparent substrate 16 and atransparent substrate 17 opposing to each other are coated with asolution for the alignment layer, the coated films are subjected to analignment process in the same direction to form alignment layers (notshown), and a sealing member, not shown, are employed to form a cell.Further, in the cell, a solution of liquid crystal monomer is injectedto form a liquid crystal monomer layer in which the direction of liquidcrystal molecules are uniformly aligned in parallel with the substratesin the cell. The liquid crystal monomer layer is exposed to ultravioletrays to be solidified, whereby a liquid crystal polymer layer 12 inwhich the alignment direction of liquid crystal molecules are fixed isformed and thus a phase plate 220 is obtained. Accordingly, a liquidcrystal element 200 in which a liquid crystal cell 210 and a phase plate220 are laminated, is obtained.

[0074] Here, the slow axis direction (a direction providing theextraordinary refractive index n_(e) (lc)) of the liquid crystal layer11 made of a nematic liquid crystal, is at 45° to Y-axis as thepolarization direction of incident light in FIG. 8 and FIG. 9. Further,the fast axis direction (a direction providing the ordinary refractiveindex n.) of the phase plate 220 made of the liquid crystal polymerlayer 12 having an ordinary refractive index of n_(o) and anextraordinary refractive index of n_(e) (n_(o)<n_(e)), is formed at anangle θ to the slow axis direction of the liquid crystal layer 11.

[0075] Here, the retardation value described in this embodiment meansthe difference between optical path of a polarized light polarized inthe fast axis direction of the liquid crystal layer 11, and optical pathof a polarized light polarized in the slow axis direction of the liquidcrystal layer 11. Therefore, a negative retardation value may exist.

[0076] Here, the angle θ between the fast axis direction of the phaseplate and the slow axis direction of the liquid crystal layer 11 ispreferably formed to be at most 45°. If the angle θ is at most 45°, theretardation value R of the phase plate becomes a negative valuecancelling the retardation value R_(v) present when the voltage isapplied. If the angle 3 exceeds 45°, R becomes a positive value and itbecomes difficult to cancel R_(v).

[0077] Usually, the fast axis direction of the liquid crystal polymerlayer 12 is made to be the same (θ=0) as the slow axis direction of theliquid crystal layer 11. Namely, the fast axis direction of the liquidcrystal polymer layer 12 is at an angle of 45° to the polarizationdirection of incident light.

[0078] Here, the thickness dlc of the liquid crystal layer 11 is made0.5λ/Δn(1c) so that for the linearly polarized light polarized in thedirection of Y-axis and having a wavelength λ, the retardation value ofthe liquid crystal cell 210 becomes approximately λ/2 when the voltagefrom AC power source 19 is not applied. Here,Δn(1c)=n_(e)(1c)−n_(o)(1c).

[0079] The transparent electrodes 13 and 14 of the liquid crystal cell210 thus obtained, are applied with AC is voltage of rectangular waveshaving a voltage amplitude of V from the AC power source 19, whereby theretardation value of the liquid crystal cell 210 is decreased to be adefinite value R_(v) but not 0. In order to make the retardation valueof the liquid crystal element zero by applying a voltage having anamplitude V, only the retardation value R of the phase plate 220 has tobe adjusted (to be −R_(v)) to cancel the above retardation value R_(v).Namely, in a case where the fast axis direction of the liquid crystalpolymer layer 12 coincides with the slow axis direction of the liquidcrystal layer 11, only the thickness d of the liquid crystal polymerlayer 12 has to be made R_(v)/Δn. Here, Δn=n_(e)−n_(o).

[0080] If the retardation value R_(v) is large, the retardation value ofthe liquid crystal cell 210 may be adjusted to be λ/2+R_(v) in advanceconsidering the reduction amount R_(v) since the retardation value ofthe liquid crystal element when the voltage is not applied becomes avalue smaller than λ/2 by R_(v).

[0081] By arranging in the thus formed liquid crystal element 200, apolarizer for transmitting only the linearly polarized light polarizedin the direction of X-axis at the light output side of the liquidcrystal element 200, when a linearly polarized light polarized in thedirection of Y-axis and having a wavelength λ is incident into theliquid crystal element 200, an optical attenuator is formed. By thisconstruction, the light is almost entirely transmitted when the voltageis not applied, and the light is shuttered by the polarizer when thevoltage is applied (applied voltage V), whereby an optical attenuatorhaving a high extinction ratio corresponding to the extinction ratio ofthe polarizer can be achieved, such being preferred.

[0082] The above description has been made with respect to a case wherethe retardation value of the liquid crystal element is substantially λ/2when the applied voltage zero, and where it is zero when the appliedvoltage is V. However, the construction may be such that the retardationvalue may be a value different from λ/2 or zero.

[0083] The range of the applied voltage to the liquid crystal cell 210is from V₁ to V₂ (V₁≠V₂), and in this voltage range, the retardationvalue changes from R₁ to R₂. In order to obtain an optical attenuatorhaving a high extinction ratio at a given voltage V within the abovevoltage range, a retardation value R_(v) of the liquid crystal cell 210produced at the voltage V and the retardation value R of the phase plate220, satisfy a relation that R+R_(v)=m×λ/2 (m: integer) for a linearlypolarized incident light having the same wavelength λ. Here, R_(v)satisfies a relation that R₁≧R_(v)≧R₂.

[0084] Here, when m is an odd number, the polarization direction of alinearly polarized light transmitted through the polarizer should bearranged to be perpendicular to the polarization direction of thelinearly polarized incident light to the liquid crystal element. On theother hand, when m is an even number, the polarization direction of thelinearly polarized light transmitted through the polarizer should bemade coincide with the polarization direction of the linearly polarizedincident light to the liquid crystal element. By such constructions, theintensity of transmitted light becomes minimum at the applied voltage Vin the range of from V₁ to V₂ and thus an optical attenuator having ahigh extinction ratio is realized. Since the dependency of theextinction ratio on wavelength becomes smaller as the absolute value ofR+R_(v) becomes smaller, usually m=0 is preferred but m=±1 or ±2 arealso acceptable.

[0085] Further, the liquid crystal element preferably has a constructionthat liquid crystal to be employed in the liquid crystal element is anematic liquid crystal, the alignment direction of liquid crystalmolecules is a parallel alignment in which the liquid crystal moleculesare uniformly aligned in an predetermined direction between thetransparent substrates when the voltage is not applied, the phase platesatisfies a relation R+R_(v)=0, and the fast axis direction of the phaseplate is at an angle of within 45° to the slow axis direction of theliquid crystal layer.

[0086] Further, a transparent substrate 16 is interposed between theliquid crystal layer 11 and the liquid crystal polymer layer 12 in FIG.8. However, the construction may be such that the transparent electrode14 and an alignment layer is formed on the liquid crystal polymer layer12 on the transparent substrate 17 without employing the transparentsubstrate 16, to form a cell and the liquid crystal layer is formedthereafter.

[0087]FIG. 8 shows an example of the construction employing a phaseplate made of a liquid crystal polymer. However, a phase plate made of abirefringent crystal such as quartz may also be employed. In such acase, a quartz waveplate on which a transparent electrode is formed maybe employed as the transparent substrates 16 of the liquid crystal cellinstead of the liquid crystal polymer layer 12 and the transparentsubstrates 16 and 17 in FIG. 8, whereby the size of the liquid crystalelement can be reduced.

[0088] Further, FIG. 8 shows a construction employing transparentelectrodes 13 and 14 as the electrodes of the liquid crystal cell.However, one of the electrodes may be a light-reflective electrode madeof e.g. gold or aluminum to form a reflective crystal cell. In thiscase, the thickness of the liquid crystal layer can be a half of that ofthe transmissive type since the light goes and returns in the liquidcrystal layer, which may lead to realize low voltage driving and quickresponse.

[0089] Liquid crystal to be employed is not limited to a nematic liquidcrystal, and it may be a ferroelectric liquid crystal, anantiferroelectric liquid crystal or the like. Further, with respect tothe alignment of liquid crystal molecules, besides the parallelalignment in which the alignment directions of alignment layers of thetransparent substrates 14 and 15 are the same, a twisted alignment inwhich the alignment directions are at a specific angle to each other sothat the alignment of liquid crystal molecules is twisted around an axisin the direction of the thickness of the liquid crystal layer. Further,depending on an aligning process for the alignment layer and theselection of the liquid crystal material, a vertical alignment in whichthe alignment direction of the liquid crystal molecules is perpendicularto the surface of the transparent substrates, or a so-called hybridalignment structure in which the alignment direction of liquid crystalmolecules is perpendicular to the surface of one transparent substrateand the direction of liquid crystal molecules is in parallel to thesurface of the other transparent substrate surface, may also beemployed. The liquid crystal material may be such one having aretardation value changeable by the application of a voltage, and theliquid crystal has only to have an alignment property. Among theseliquid crystals, a nematic liquid crystal is preferably employed sincestable liquid crystal alignment can be obtained.

Second Embodiment of the Liquid Crystal Element

[0090]FIG. 10 is a side view showing an example of the construction ofthe second embodiment of the liquid crystal element of the presentinvention comprising a multi-layer diffraction type polarizers 110 and120 combined with a liquid crystal element 200.

[0091] It is preferred to constitute a complex type liquid crystalelement comprising a multi-layer diffraction type polarizer described inthe first and the second embodiments of the multi-layer diffraction typepolarizer laminated on a surface of at least one of the transparentsubstrates of the liquid crystal cell described in the first embodimentof the liquid crystal element, since reduction of the size of the deviceand a stable extinction ratio can thereby be obtained.

[0092] As shown in FIG. 10, the fast axis direction (the direction whichprovides the ordinary refractive index) of birefringent material layersconstituting the multi-layer diffraction type polarizers 110 and 120bonded to the liquid crystal element 200 by employing a transparentadhesive (not shown), is formed at an angle of 45° in a case of themulti-layer diffraction type polarizer 110, and at an angle of 135° in acase of the multi-layer diffraction type polarizer 120 with respect toX-axis in an XY plane in FIG. 4. Namely, two diffraction gratings in themulti-layer diffraction type polarizer 110 are linear type polarizers,and longitudinal directions of their gratings are at an angle of 45° toX-axis direction. Two diffraction gratings in the multi-layerdiffraction type polarizer 120 are also linear type polarizers, andlongitudinal directions of their gratings are at an angle of 135° toX-axis direction.

[0093] When light having a wavelength λ is incident into the complextype liquid crystal element 300 having such structure from a side of themulti-layer diffraction type polarizer 110, a first linearly polarizedlight polarized at an angle of 45° to X-axis is transmitted withoutbeing diffracted by the multi-layer diffraction type polarizer 110,while a second linearly polarized light polarized in a direction at anangle of 135° to X-axis is transmitted after being diffracted by themulti-layer diffraction type polarizer 110, and thereafter, both of themare incident into the liquid crystal element 200.

[0094] When a voltage is not applied to the liquid crystal cell 210 ofthe liquid crystal element 200 (refer to FIG. 8), namely, when V₁=0, theliquid crystal cell 210 functions as a phase plate producing a phasedifference π for the first and second incident linearly polarizedlights. Namely, since it functions as a ½ waveplate having a retardationvalue R₁=λ/2, the straightly transmitted light not diffracted by themulti-layer diffraction type polarizer 110 is converted to a linearlypolarized light polarized at an angle of 135° to X-axis, and thetransmitted light diffracted by the multi-layer diffraction typepolarizer 110 is converted to be a linearly polarized light polarized atan angle of 225° to X-axis.

[0095] As a result, the straightly transmitted light not diffracted bythe multi-layer diffraction type polarizer 110 is incident into themulti-layer diffraction type polarizer 120 as an ordinary polarizedlight, and straightly transmitted without being diffracted. On the otherhand, the light diffracted by the multi-layer diffraction type polarizer110 is incident into the multi-layer diffraction type polarizer 120 asan extraordinary polarized light, in which the incident light isdiffracted. Accordingly, among these types of incident light in thecomplex type liquid crystal element 300, the first linearly polarizedlight is straightly transmitted without being diffracted, and the secondlinearly polarized light polarized in a direction perpendicular to thepolarization direction of the first linearly polarized light, isdiffracted and output. Here, since the longitudinal direction of thediffraction grating constituting the multi-layer diffraction typepolarizer 110 is different from that of the diffraction gratingconstituting the multi-layer diffraction type polarizer,120,multi-diffraction light generated does not superpose the straightlytransmitted light on the optical axis.

[0096] Further, by adjusting the retardation value R of a phase plate220 so as to cancel the retardation value R_(v) remaining in the liquidcrystal layer 11 when a specific voltage V is applied to the liquidcrystal cell 210 of the liquid crystal element 200 (refer to FIG. 8),the total of retardation values of the liquid crystal layer and thephase plate becomes R+R_(v)=m×λ (m: integer) and the incident light isnot changed in its phase difference and is output maintaining thepolarization.

[0097] As a result, the straightly transmitted light not diffracted bythe multi-layer diffraction type polarizer 110 is incident into themulti-layer diffraction type polarizer 120 as an extraordinary polarizedlight, in which the light is diffracted. On the other hand, thetransmitted light diffracted by the multi-layer diffraction typepolarizer 110 is incident into the multi-layer diffraction typepolarizer 120 as an ordinary polarized light and is not diffracted.Accordingly, among these types of incident light in the complex typeliquid crystal element 300, both the first linearly polarized light andthe second linearly polarized light are diffracted and output. Namely,the incident light is diffracted regardless of its polarization stateand not present on the optical axis of straight transmission.

[0098] Therefore, by switching on/off the voltage applied to the liquidcrystal cell 210, the straightly transmitted light is separated from thediffracted light. In FIG. 5, by disposing the complex type liquidcrystal element 300 instead of the multi-layer diffraction typepolarizer 100, a polarizer type switching device having a highextinction ratio can be realized. Further, by applying a predeterminedvoltage without switching on/off the applied voltage, the intensity ofstraightly transmitted light having a predetermined polarizationdirection can be adjusted to be a predetermined intensity, whereby theliquid crystal cell can function as a voltage variable opticalattenuator.

[0099] This embodiment shown in FIG. 10 has a construction that the fastaxes of the birefringent material layers of the multi-layer diffractiontype polarizer 110 and the multi-layer diffraction type polarizer 120are perpendicular to each other. However, the construction may be thatthey are in parallel with each other. In this case, the transmittance ofthe straightly transmitted light becomes minimum when the voltage is notapplied to the liquid crystal cell (V₁=0) and maximum when the voltageis applied (V).

[0100] In FIG. 10, an example that the polarizing diffraction typepolarizers are disposed at light input and output sides of the liquidcrystal element 200. In a case where only a linearly polarized lightcomponent polarized in the same direction as the transmittablepolarization of the input side polarizer is incident, a polarizingdiffraction type polarizer needs to be disposed only at the light outputside.

[0101]FIG. 12 shows an example of another construction of the complextype liquid crystal element comprising the multi-layer diffraction typepolarizer 120 and the liquid crystal element 200 in combination. At thelight input side of the liquid crystal element 200, a polarizationconversion device 25 is disposed, which comprises a prism in which apolarization separator film 22 and a total reflective mirror 23 areformed, and a ½ waveplate 24 is bonded to the prism.

[0102] Among two types of linearly polarized incident light on thepolarization conversion device 25, one type of linearly polarized lightis transmitted through the polarization separator film 22. Another typeof linearly polarized light polarized in a direction perpendicular tothe one type of linearly polarized light is reflected by thepolarization separator film 22 and the total reflective mirror film 23to be introduced to the ½ waveplate 24 at which the polarization plane(polarization direction) is rotated by 90° so that it becomes linearlypolarized light polarized in the same direction as the one type oflinearly polarized light and incident into the complex type liquidcrystal element 310. As a result, a switching device or an opticalattenuator having low insertion loss can be realized regardless of thestate of polarization of incident light.

[0103] Further,,by patterning the transparent electrode layer 13 or 14of the liquid crystal cell 210 and applying a voltage independently toeach of the patterned electrodes, the spatial distribution of thetransmittance can be adjusted in accordance with the patterned shape.

[0104] Further, in the polarizing diffraction type polarizer of thepresent invention, since the temperature rise of the liquid crystalelement caused by light absorption is little, stable light attenuationcan be obtained even under high intensity incidence of light as comparedwith a conventional light absorption type polarizer which absorbs aparticular polarization component.

[0105] In this embodiment, an optical attenuator of high extinctionratio is realized by combining a multi-layer diffraction type polarizercomprising laminated diffraction type polarizers with the liquid crystalelement. However, it is acceptable to combine a single polarizer withthe liquid crystal element. In this case, the maximum extinction ratiois decreased and the wavelength bandwidth is further decreased.

[0106] As described above, by disposing a polarizing diffraction typepolarizer at at least one of the light input side and light output sideof the liquid crystal element to form an integrated structure, reductionof the device size and stable performance can be obtained, such beingpreferred.

Third Embodiment of the Liquid Crystal Element

[0107] An embodiment of a liquid crystal element of the presentinvention which comprises a liquid crystal cell, and a phase platehaving an organic material layer producing a phase differencecorresponding to a retardation value of substantially λ/4 for linearlypolarized incident light, wherein the fast axis direction of the liquidcrystal cell is at an angle of about 45° to the fast axis direction ofthe phase plate, will be described.

[0108] The liquid crystal element of the present invention will bedescribed employing a side view of FIG. 8. The structure of the liquidcrystal cell is the same as that of the liquid crystal cell 210 of thefirst embodiment except that the structure of the phase plate 220 isdifferent and that incident light is transmitted from a side of theliquid crystal cell 210 toward the phase plate 220.

[0109]FIG. 13 is a view illustrating a coordinate system showing therelation of the fast and slow axes of the liquid crystal and the phaseplate constituting the liquid crystal element of the present inventionand polarization directions.

[0110] In FIG. 8, the phase plate 220 comprises a liquid crystal polymerlayer, which is obtainable by coating one side of each of thetransparent substrates 16 and 17 opposed to each other with a layer toform an alignment layer, applying each of the layers an alignmenttreatment in the same direction to form alignment layers (not shown),and employing a seal member to form a cell. A solution of liquid crystalmonomer is injected in the cell to form a liquid crystal monomer layerin which the direction of liquid crystal molecules are aligned in adirection parallel with the substrate surfaces in the cell, andirradiating ultraviolet rays to polymerize and solidify the liquidcrystal monomer layer, whereby a liquid crystal polymer layer in whichthe alignment direction of liquid crystal molecules is fixed, isobtainable.

[0111] Here, the construction is made so that the fast axis direction(direction of ordinary refractive index n_(o) (1c)) of the liquidcrystal cell comprising the liquid crystal layer 11 is, for example, atan angle of 45° to the fast axis direction (direction of ordinaryrefractive index n_(o)) of the phase plate comprising the liquid crystalpolymer layer 12 having an ordinary refractive index n_(o) and anextraordinary refractive index n_(e) (n_(o)<n_(e)).

[0112] In FIG. 13, it is determined so that the fast axis direction ofthe liquid crystal layer 11 is at an angle of 135° to X-axis being thepolarization direction of incident light, and light is incident from aside of the liquid crystal cell 210. The angle between the fast axisdirection of the liquid crystal cell and the fast axis direction of thephase plate may deviate from 45° as long as the effect of the presentinvention is maintained, and it may be from 40° to 50°.

[0113] Here, the thickness dlc of the liquid crystal-layer 11 is made tobe 0.5λ/(n_(e)(lc)−n_(o)(1c)) so that the retardation value of theliquid crystal cell 210 becomes, for example, λ/2 for linearly polarizedincident light, polarized in X-axis direction, having a wavelength whenthe voltage is not applied, and the retardation value of the phase plate220 is made to be substantially λ/4. Here, the retardation value of thephase plate 220 may deviate from λ/4 so long as it is within a range inwhich there is an effect of maintaining the linearity of the linearlypolarized output light from the liquid crystal element. Further, it maybe a value an odd number times greater than λ/4.

[0114] When the rectangular AC voltage applied from the AC power source19 to the transparent electrodes 13 and 14 of the liquid crystal cell210 thus obtained is increased, the retardation value R of the liquidcrystal cell 210 is changed to 0 from λ/2 which is the value when thevoltage is not applied.

[0115] In a liquid crystal polymer, since the alignment direction ofliquid crystal molecules is in parallel with the substrate surfaces, theretardation value does not change remarkably even if the incident angleas an angle at which the propagation direction of the incident lightcrosses the normal line of the phase plate, is tilted from 0° to about20°, and the liquid crystal polymer functions as a stable λ/4 phaseplate for incident light having a wavelength λ.

[0116] In the liquid crystal cell 210, since the rotation angle θbetween the polarization direction of output light and that of incidentlight is represented by θ=180×R/λ where R is the retardation value ofthe liquid crystal cell 210, θ is decreased from 90° to 0° as theapplied voltage to the liquid crystal cell 210 is increased.

[0117] As described above, it is preferred that the liquid crystalemployed for the liquid crystal element is a nematic liquid crystal, thealignment directions of liquid crystal molecules at both substrates eachhaving electrodes are aligned in parallel in the same predetermineddirection when the voltage is not applied, the phase plate has a liquidcrystal polymer, and the liquid crystal cell and the phase plate areintegrally formed. The reasons of the above construction are that thesize of the device is thereby reduced, and that the angle at which thefast axis direction of the liquid crystal cell crosses the fast axisdirection of the phase plate is thereby fixed and the stability of theoptical performance is improved.

[0118] Further, in a case where light having a center wavelength λ and acertain bandwidth of the wavelength, it is preferred to employ a phaseplate constituted by laminating two liquid crystal polymer layers havingretardation values and fast axis directions different from each other.By the lamination, the dependency of the retardation value on wavelengthcan be reduced, and deterioration of the linearity of the outputlinearly polarized light can be reduced.

[0119]FIG. 14 is a side view showing another example of the constructionof the liquid crystal element of the present invention employing such aphase plate.

[0120] The liquid crystal cell 210 has the same construction as in FIG.8, in which a liquid crystal polymer layer 31 having a retardation valueof, for example, λ/2 and a liquid crystal polymer layer 32 having aretardation value of, for example, λ/4 are formed one side of thetransparent substrates 16 and 17 respectively, and they are bonded so asto sandwich the liquid crystal polymer layer by employing an adhesive 33comprising a homogeneous refractive index transparent material tothereby form the phase plate 230.

[0121] Here, the alignment directions of liquid crystal molecules of theliquid crystal polymer layers are preferably aligned such that in thecoordinate system shown in FIG. 13, with respect to the polarizationdirection of the incident light polarized in the direction of X-axis,the fast axis of the liquid crystal polymer layer 31 is at an angle of,for example, 30°, and the fast axis of the liquid crystal polymer layer32 is at an angle of, for example, −30°. Here, the sign of the angle isdefined so that the rotation angle from +X axis direction towards +Yaxis direction has a positive sign. Here, the values λ/2 and λ/4 maydeviate from these values so long as they are within ranges in whichthere is an effect of maintaining the linearity of the output linearlypolarized light from the liquid crystal element. The 30° and −30° mayeach has a tolerance of ±5° from them.

[0122] Further, the alignment directions of liquid crystal molecules ofthe liquid crystal polymer layers may be aligned so that with respect tothe polarization direction of the incident light polarized in thedirection of X-axis, the slow axis of the liquid crystal polymer layer31 is at an angle of about 30°, and the slow axis of the liquid crystalpolymer layer 32 is at an angle of about −30°.

[0123] By thus constituting a liquid crystal element 400 comprising thephase plate 230 and the liquid crystal cell 210 integrally formed, highlinearity of linearly polarized output light can be maintained even iflight having a certain bandwidth in the wavelength is incident.

[0124] In the above example, an example of employing a liquid crystalpolymer as the organic film of the phase plate having a function ofgenerating phase difference, is shown. However, an organic film such aspolycarbonate film stretched in one direction to impart a birefringenceproperty may also be employed as the organic film.

[0125] Further, a phase plate, not shown, having a different opticalaxis from the liquid crystal layer may be laminated on the liquidcrystal cell 210, whereby the voltage variable rotation angle of theoutput linearly polarized light can be adjusted.

[0126] Further, the multi-layer diffraction type polarizer of thepresent invention may be bonded to be fixed to the transparent substrate17 at the light input side of the liquid crystal element 400, wherebyonly incident light having high linearity can be straightly transmitted,and thus it is easy to maintain the linearity of the output light fromthe liquid crystal element.

[0127] Now, Examples will be described.

EXAMPLE 1

[0128] The multi-layer diffraction type polarizer of the presentinvention will be described employing FIG. 1. On respective one side ofthe transparent substrate 4 and the transparent substrate 5 made ofglass substrates, liquid crystal polymer layers each having an ordinaryrefractive index n₀=1.55 and an extraordinary refractive indexn_(e)=1.70 were formed as a birefringent material layer, and appliedwith photolithography and etching techniques to form linear diffractiongratings 1 and 2. The grating pitches p₁ and p₂ of the diffractiongratings 1 and 2 were 20 μm and 40 μm respectively, longitudinaldirections of the gratings were in parallel with each other, and thedepths of the convex portions of the liquid crystal polymer layers ofthe diffraction gratins 1 and 2, namely the step heights d₁ and d₂, were4.8 μm and 5.6 μm respectively.

[0129] Further, concavo-convex portions of the liquid crystal polymerlayers processed to have a concavo-convex form were filled with ahomogeneous refractive index transparent material 3 made of atransparent resin having a refractive index of n_(s)=1.55, and atransparent substrate 6 made of a glass substrate was laminated thereonto produce a multi-layer diffraction type polarizer 100 as a laminationof a polarizing diffraction grating constituted by the diffractiongrating 1 and a polarizing diffraction grating constituted by thediffraction grating 2. Here, the liquid crystal polymer layer was formedby injecting a solution of liquid crystal monomer into the spacingbetween the substrates each having an alignment layer (already subjectedto an aligning treatment), and irradiating ultraviolet rays to theliquid crystal monomer to polymerize and solidify it. Further,antireflection films are formed in the interface between the transparentsubstrates 4 and 6, and air.

[0130] When a parallel light having a wavelength band of 1400 to 1700 nmwas incident into the multi-layer diffraction type polarizer 100, anordinary polarized light was hardly diffracted and 97% of the incidentlight was straightly transmitted. An extraordinary polarized lightpolarized in the direction perpendicular to the ordinary polarized lightwas almost entirely diffracted and at most 0.05% was straightlytransmitted. As shown in FIG. 5, the transmitted light through themulti-layer diffraction type polarizer 100 was focused on the focalplane of a condenser lens 7 so that only the straightly transmittedlight on the optical axis was focused to form an image at the coreportion, not shown, of an optical fiber. As a result, an isolator havinga high extinction ratio as a ratio of the extraordinary polarized lightto the ordinary polarized light, of at most −30 dB in a wavelength bandof 1400 to 1700 nm, was obtained.

EXAMPLE 2

[0131] A liquid crystal element 200 of the present invention will bedescribed employing FIG. 8. A nematic liquid crystal having an ordinaryrefractive index of n_(o)(lc)=1.50 and an extraordinary refractive indexof n_(e)(lc)=1.66 was sandwiched between transparent substrates 15 and16 having transparent electrodes 13 and 14 formed on one side of themrespectively, to produce a liquid crystal cell 210 comprising a liquidcrystal layer 11 having a thickness d (lc) of 5 μm. The slow axisdirection of the liquid crystal layer 11 was at 45° to Y-axis directionshown in FIG. 9 and in parallel with the substrates.

[0132] The retardation value of the liquid crystal cell 210 was 0.8 μm(R₁) for light having a wavelength of 1.55 μm. in a state that thevoltage is not applied to the transparent electrodes 13 and 14, and whena linearly polarized light polarized in Y-axis direction was incidentinto the liquid crystal cell 210, the polarization direction of thelinearly polarized light output from the liquid crystal cell 210 was inX-axis direction. Further, under the condition of applying a rectangularAC voltage having a voltage amplitude of 5 V, the retardation valueR_(v) of the liquid crystal cell 210 was 0.128 μm (R₂).

[0133] Further, a phase plate 220 was constituted wherein a liquidcrystal polymer layer 12 sandwiched between the transparent substrates16 and 17 had an ordinary refractive index of n_(o)=1.55, anextraordinary refractive index of n_(e)=1.59 and a thickness d of 3.2μm. The liquid crystal element 200 was constituted by the phase plate220 and the liquid crystal cell 210. Here, if the fast axis direction ofthe phase plate 220 is made coincide with the slow axis direction of theliquid crystal layer 11, namely if θ=0 in FIG. 9, the retardation valueR of the phase plate 220 constituted by the liquid crystal polymer layer12 becomes −0.128 μm and cancels the retardation value 0.128 μm presentin the liquid crystal layer 11 when the applied voltage is 5 V. Namely,this corresponds to m=0 in m×λ=0.

[0134] Further, the same multi-layer diffraction type polarizer 120 asExample 1 was bonded to be fixed to the light output side of the liquidcrystal element 200 to form a complex type liquid crystal element 310(having a construction without a polarized light conversion element 25)shown in FIG. 12. Here, the polarization direction of diffracted lightby the polarizer 200 is designated as Y-axis.

[0135] On the complex type liquid crystal element 310 thus produced, alinearly polarized parallel light polarized in Y-axis direction andhaving a wavelength of 1.55 μm was incident and the output light wasfocused by a condenser lens into an optical fiber, not shown.

[0136] The voltage amplitude applied to the liquid crystal layer of theliquid crystal element was changed from 0 to 5 V. An example of thechange of the extinction ratio defined as the ratio of light intensityI(V)/I(0) provided that light intensity transmitted through the opticalfiber is designated as I(V), is shown as ◯ in FIG. 11. In FIG. 11, theextinction ratio becomes larger as the value on the ordinate axis issmaller (as the position is lower). Further, as a Comparative Example, achange of the extinction ratio in a case of a complex type liquidcrystal element without having the phase plate 220 is shown by □.

[0137] The extinction ratio was at most −12 dB in the case of the liquidcrystal element without employing the phase plate. However, in the caseof the construction of this Example employing the phase plate, a highextinction ratio of −40 dB was obtained.

EXAMPLE 3

[0138] The liquid crystal element of the present invention will bedescribed employing FIG. 14. A nematic liquid crystal having an ordinaryrefractive index of n₀ (1c)=1.50 and an extraordinary refractive indexof n_(e) (lc)=1.66 was employed to form a liquid crystal layer 1 havinga thickness d (1c) of 4.5 μm. Alignment layers formed on the substrateswere subjected to an aligning treatment so that the fast axis directionof the liquid crystal cell constituting the liquid crystal elementbecame 135° with respect to X-axis in FIG. 14.

[0139] Further, both of the liquid crystal polymer layers 31 and 32 asphase plates were formed to have an ordinary refractive index of n₀=1.55and an extraordinary refractive index of n_(e)=1.65. The thicknesses ofthe liquid crystal polymer layers were 7.7 μm and 3.85 μm respectivelyso that their retardation values for the central wavelength 1550 nm ofthe wavelength band from 1400 nm to 1700 nm, became λ/2 and λ/4respectively. Here, the liquid crystal polymer layers 31 and 32 werebonded so that their fast axis directions are at 30° and −30°respectively with respect to X-axis as the polarization direction ofincident light, namely, so that their fast axis directions were at anangle of 60° to each other by employing an adhesive 33 made of ahomogeneous refractive index transparent material.

[0140] The phase plate 230 comprising a lamination of the liquid crystalpolymer layers 31 and 32 thus obtained, is a phase plate having aretardation value of substantially λ/4 for the incident light, and itsfast axis direction and the fast axis direction of the liquid crystalcell forms an angle of 45°.

[0141] On the liquid crystal element, a linearly polarized light havinga wavelength of from 1400 nm to 1700 nm and polarized in X-axisdirection, is incident. When the voltage was not applied, theretardation value of the liquid crystal cell 210 was 0.72 μm and theoutput light from the liquid crystal element 400 became a linearlypolarized light polarized in a direction about 150° rotated from thepolarization direction of the incident light (the angle between bothdirections is about 30°). Further, the retardation value of the liquidcrystal cell 210 was at most 0.05 μm when a rectangular wave AC voltagehaving a voltage amplitude of at least 10 V was applied, and the outputlight from the liquid crystal element 400 became a linearly polarizedlight polarized in a direction about 60° rotated from the polarizationdirection of the incident light.

[0142] Here, the ellipticity (a ratio a/b of the minor axis amplitude tothe major axis amplitude b of the output elliptically polarized light)showing the linearity of the output linearly polarized light, showed ahigh linearity of at most 0.01 in a wavelength band of from 1400 nm to1700 nm, and a polarization rotator was obtained, which can rotate thepolarization direction of the output light from 150° to 60° inaccordance with the applied voltage of from 0 to 10 V.

Industrial Applicability

[0143] As described above, the multi-layer diffraction type polarizer ofthe present invention has a high extinction ratio for incident lighthaving a wide wavelength band. By employing the multi-layer diffractiontype polarizer of the present invention, a high performance isolator canbe obtained.

[0144] Further, by employing a liquid crystal element of the presentinvention, the output light intensity is gradually decreased as theapplied voltage is increased, whereby a voltage variable type opticalattenuator having a high extinction ratio even at a low voltage of about5V can be realized.

[0145] Further, by turning on/off the applied voltage, a switchingelement having a high extinction ratio can be obtained.

[0146] Further, by forming a complex type liquid crystal element of thepresent invention comprising the multi-layer diffraction type polarizerand the liquid crystal element integrally formed, temperature rise ofthe liquid crystal layer is small even if high intensity light isincident since the element absorbs little light. As a result, an opticalattenuator providing a stable extinction ratio can be realized.

[0147] Further, by employing the liquid crystal element of the presentinvention, the state of linear polarization is maintained regardless ofthe variation of the incident angle and the wavelength of linearlypolarized incident light on the liquid crystal element, whereby apolarization rotator is obtained which can rotate the polarizationdirection in accordance with the magnitude of the voltage applied to theliquid crystal cell constituting the liquid crystal element.

[0148] The entire disclosures of Japanese Patent Application No.2001-254700 filed on Aug. 24, 2001, Japanese Patent Application No.2001-256301 filed on Aug. 27, 2001 and Japanese Patent Application No.2001-296605 filed on Sep. 27, 2001 including specifications, claims,drawings and summaries are incorporated herein by reference in theirentireties.

What is claimed is:
 1. A multi-layer diffraction type polarizercomprising a lamination of at least two polarizing diffraction gratingseach comprising a birefringent material, wherein the diffractiongratings each straightly transmits incident light having a firstpolarization direction without functioning as a polarizer, and diffractsincident light having a second polarization direction perpendicular tothe first polarization direction by functioning as a polarizer.
 2. Themulti-layer diffraction type polarizer according to claim 1, whereineach of the polarizing diffraction gratings comprises a birefringentmaterial layer formed on a transparent substrate and having an ordinaryrefractive index of no and an extraordinary refractive index of n_(e)(n_(o)≠n_(e)), the birefringent material layer being processed to have aperiodical concavo-convex shape having a step height of d in crosssection, a homogeneous refractive index transparent material having arefractive index equal to n_(o) or n_(e) is filled in at least theconcave portions, and the retardation value |n_(e)−n_(o)|×d is (m+½)times (m is zero or a positive integer) the wavelength λ of the incidentlight.
 3. The multi-layer diffraction type polarizer according to claim1, wherein the step heights d of the polarizing diffraction gratings aredifferent from each other.
 4. The multi-layer diffraction type polarizeraccording to claim 2, wherein the step heights d of the polarizingdiffraction gratings are different from each other.
 5. A liquid crystalelement comprising: a liquid crystal cell comprising transparentsubstrates having electrodes and a liquid crystal layer sandwichedbetween them, the liquid crystal cell having a retardation value for alinearly polarized light having a wavelength of λ incident andtransmitting through the liquid crystal cell, the retardation valuechanging from R₁ to R₂ (R₁>R₂>0) when the voltage applied between theelectrodes is changed from V₁ to V₂ (V₁≠V₂) ; and a phase plate having aretardation value R for a linearly polarized light having a wavelengthof λ, the retardation value R satisfying a relation R+R_(v)=m×λ (m:integer) with a retardation value R_(v) generated by the voltagesatisfying R₁≧R_(v)≧R₂.
 6. The liquid crystal element according to claim5, wherein the liquid crystal in the liquid crystal element is a nematicliquid crystal, and the alignment of the liquid crystal molecules is aparallel alignment in which the liquid crystal molecules are aligned inparallel in one direction between the transparent substrates when thevoltage is not applied, the first phase plate satisfies a relationR+R_(v)=0, and the fast axis direction of the first phase plate iswithin an angle of 45° with respect to the slow axis direction of theliquid crystal layer.
 7. The liquid crystal element according to claim5, which further comprises a polarizing diffraction grating comprising abirefringent material at at least one of the light input side and thelight output side of the liquid crystal element, wherein the diffractiongrating straightly transmits incident light having a first polarizationdirection without functioning as a polarizer, and diffracts incidentlight having a second polarization direction perpendicular to the firstpolarization direction by functioning as a polarizer.
 8. The liquidcrystal element according to claim 6, which further comprises apolarizing diffraction grating comprising a birefringent material at atleast one of the light input side and the light output side of theliquid crystal element, wherein the diffraction grating straightlytransmits incident light having a first polarization direction withoutfunctioning as a polarizer, and diffracts incident light having a secondpolarization direction perpendicular to the first polarization directionby functioning as a polarizer.
 9. A liquid crystal element having a λ/4phase plate, comprising: a liquid crystal cell comprising substrateshaving electrodes and a liquid crystal layer sandwiched between them,the liquid crystal cell having a retardation value changeable forlinearly polarized incident light having a wavelength of λ depending onthe magnitude of a voltage applied between the electrodes; and a λ/4phase plate producing a phase-shift corresponding to a retardation valueof substantially λ/4 for the linearly polarized incident light, the λ/4phase plate having an organic thin film and the alignment of moleculesconstituting the organic thin film is in parallel with the plane of thephase plate; wherein the fast axis direction of the phase plate is at anangle of about 45° to the fast axis direction of the liquid crystalcell.
 10. The liquid crystal element having a λ/4 phase plate accordingto claim 9, wherein the phase plate comprises a liquid crystal polymeras the organic thin film, and the phase plate and the liquid crystalcell are integrally formed.
 11. The liquid crystal element having a λ/4phase plate according to claim 10, wherein the phase plate comprises atleast two liquid crystal polymer layers, the retardation values of thetwo liquid crystal polymer layers are different from each other, and thefast axis directions or the slow axis directions of the two liquidcrystal polymer layers are different from each other.
 12. The liquidcrystal element having a λ/4 phase plate according to claim 11, whichcomprises the liquid crystal cell, a first liquid crystal polymer layerand a second liquid crystal polymer layer arranged in this order fromlight input side, wherein with respect to the center wavelength λ ofincident light, the retardation value of the first liquid crystalpolymer layer is substantially λ/2, and the retardation value of thesecond liquid crystal polymer layer is substantially λ/4; and withrespect to the polarization direction of the incident light, the fastaxis direction of the first liquid crystal polymer layer and the fastaxis direction of the second liquid crystal polymer layer are about 30degrees and about −30 degrees respectively, or otherwise, the slow axisdirection of the first liquid crystal polymer layer and the slow axisdirection of the second liquid crystal polymer layer are about 30degrees and about −30 degrees respectively.